The present invention pertains generally to material feeders, and more particularly to a set rate feeder for transporting fluidized particulate material for processing.
Material feeders are commonly used to provide a continuous flow of material at a constant predetermined flow rate, for further processing. Gravimetric belt feeders discharge the material on a weight/time basis, and thereby automatically take into account any changes in the bulk density of the material. In one class of gravimetric feeders, of the continuous flow-set rate type, material is discharged into a process in a continuous stream at a predetermined rate. This class of feeder performs admirably with many types of particulate materials, but is not entirely suitable when the particulate is very finely divided to the point that the particles move freely among themselves, remaining in one mass, keeping the same volume, and taking the form of the containing vessel.
Such finely divided particulate materials or powders, as, for example, fly ash, and pulverized coal, are often referred to as fluid-like, fluidized or fluidizable, because they behave very nearly as fluids in the above-mentioned respects. When fluidized materials are to be transported in a feeder within a process they become difficult to control because of their tendency to entrain so much air that they act as liquid materials and gush out through any escape ports, such as the outlet of a bin, in a flood.
It is a principal object of the present invention to provide a process feeder which is capable of transporting fluidized particulate material at a continuous predetermined rate in a manner to control, and indeed prevent, flooding of the material.
Various types of continuous flow set rate feeders are known and in common use. In U.S. Pat. No. 3,786,961, Wahl et al. disclose a belt type continuous flow set rate gravimetric feeder in which material is discharged from the spout of a hopper onto a horizontally disposed endless belt mounted between spaced pulleys. A variable speed motor drives one of the pulleys. The belt is pivotally supported at one end by a pair of leaf springs, and pivots downwardly about a horizontal axis according to the weight of material on the belt. A load cell arranged at and beneath the opposite end of the belt senses this displacement of the belt, and generates an output signal corresponding to the displacement, and thus to the weight of the material on the belt. The output signal is supplied to circuitry for regulating the speed of the motor. In this manner, the belt speed is automatically adjusted to maintain a preset rate at which the material is discharged by the feeder.
Unfortunately, the belt feeder disclosed in U.S. Pat. No. 3,786,961 encounters the same types of problems as are experienced with all known conventional continuous flow set rate belt feeders when the particulate material is fluidized. The primary sources of these difficulties in conveying such material are the incapability of a flat belt feeder to retain a fluidized particulate without considerable runoff, and the aforementioned tendency toward flooding.
U.S. Pat. No. 4,475,669 to Wahl discloses a belt feeder which provides, among other things, a partial solution to the problem of conveying fluidized particulates through the use of a belt having a series of spaced apart cleats, wherein the cleats and the upper housing wall are almost in contact. The feeder constitutes an endless series of compartments that serve to hold the material and, to an extent, prevent runoff of the fluidized material. However, it has been found extremely difficult to uniformly fill the compartments with such material, and, hence, the desired steady pre-set rate is not readily achieved. The flooding property of fluidized materials renders a belt feeder virtually incapable of controllably transporting such materials, because the unrestrained flow simply inundates the equipment. It should be noted that screw feeders fare no better because they present a continuous open path around the helical screw shaft, and fluidized particulates will simply flush through that path.
Industrial practice often dictates that materials be pulverized, aerated and fluidized for easy transport, but reliable feeding of such materials on a belt or screw becomes so difficult for the foregoing reasons, that the anticipated ease of transport is self-defeating.
In U.S. Pat. No. 3,884,3l3 to Engels there is disclosed a feeding and measuring device for flowable solid materials, in which the device can act as a meter in the flow stream by holding material going through a weighing box such that all material has a uniform dwell time. The Engels device is not, strictly speaking, a feeder as that terminology is commonly used in the industry, because a feeder is capable of regulating the flow rate of the conveyed material over a range of values, whereas the Engels device merely accepts and discharges material at the rate it is poured into the inlet funnel. Moreover, scrapers utilized in the Engels device are incapable of containing fluidized particulate material since such material can readily flow over and around the scrapers. Thus, the Engels device cannot readily feed fluidized particulate material at a continuous pre-set rate.
Accordingly, it is another object of the present invention to provide a set rate feeder for accurately conveying and dispensing fluidized particulate materials at a controlled uniform rate despite variations in the rate at which the material is supplied to the feeder.